Liquid mixture is everywhere in the chemical industry and widely studied by researchers.
An accurate prediction of its physicochemical property is of vital importance in developing efficient process optimization. However, measurements from experiment are usually time consuming and inefficient. Furthermore, clear understanding of many of fundamental physicochemical phenomena hasn't been obtained, which restricts the development of novel products. Molecular simulation techniques have become an impressive tool to deal with these challenges during past decades. This thesis mainly applied molecular simulation to predict the physicochemical properties of industrially relevant mixtures and investigate the molecular mechanism behind observed phenomena. Among various properties, cohesive energy is the central focus, which reveals intermolecular interactions between molecules of different types. Mixture systems of two different areas of application were studied.
The first is amorphous polymer-plasticizer mixtures, which, with varying composition, correspond to plastic products of different grades for application in different areas. The most important class of plasticizers are phthalate diesters, in which di (2-ethylhexyl) phthalate (DEHP) is the most frequently used compound. However, phthalates are prone to migration loss from the host poly(vinyl chloride) (PVC), which results in the contamination of surrounding environment, gradual deterioration of plastics performance, and potential harm to human health. It has thus prompted tightening governmental regulation on their usage. With this background, we aim to address three challenges: (I) model plasticized PVC to predict its physicochemical property, (II) obtain molecular insight into plasticization and plasticizer diffusion pattern inside PVC, (III) correlate plasticizer performance -- compatibility, efficacy, and mobility -- with its molecular structure. Cohesive energy plays a central role especially in understanding plasiticzer compatibility and migration tendency.
Our modeling and simulation protocol is firstly tested on phthalates, where the simulated plasticization efficacy and thermodynamic compatibility with the host polymer agree well with all known experimental observations. Furthermore, through simulation of plasticizer diffusion pattern, we found relaxation of the alkyl side chains is a key factor in plasticizer migration. Next, we expand our simulation to a wider group of plasticizers including adipates, trimellitates, and citrates. The computed mixing enthalpy and Young's modulus again show an excellent agreement with available experimental data. Dependance of plasticizer performance on seven molecular design parameters are evaluated. The obtained relationship clearly tells us decreasing leg length or increasing branching on the leg will raise plasticizer compatibility with PVC, changing the torso group from benzene ring to alkane chain will highly improve plasticizer efficacy, and attaching three legs on the torso will decrease plasticizer mobility. As a side outcome, we also report a nontrivial chain-length dependence of the cohesive energy and solubility parameter of long-chain polymers, which is an important consideration in the calculation of these quantities using molecular simulation.
The second area is azeotropes, the separation of which in chemical processes is usually very difficult due to the same composition in vapor and liquid phases at the azeotropic point. So far, a fundamental understanding of azeotrope formation is still missing. In this thesis, we aim to address two fundamental questions: (I) the mechanism for ethanol/benzene azeotrope formation, (II) classification of different polar-polar positive azeotropes.
First, Gibbs ensemble Monte Carlo (GEMC) simulation is performed to predict the vapor-liquid equilibrium (VLE) phase diagram of ethanol/benzene, including an azeotrope point. The results match well with experiments. Free energy and cohesive energy profiles analyses are then performed. From a thorough liquid structure analysis, we conclude a three-stage mechanism for azeotrope formation: 1) formation of small ethanol clusters at low composition, 2) microscopic phase separation between ethanol and benzene, 3) isolation of benzene. This approach is then extended to four additional polar-polar mixtures (ethyl acetate/methanol, ethyl acetate/ethanol, ethanol/water, and 1-propanol/water) to obtain their VLE diagrams, which again match well with experiments. Free energy and cohesive energy analyses indicate that there are two types of mechanisms, a three-stage mechanism with weak cross-interactions (for the first two mixtures) and a three-stage mechanism with strong cross-interactions (for the last two mixtures). So far, our analyses on mixture liquid micro-structure can partially prove the existence and classification of those mechanisms.
Overall, the successful prediction in physicochemical properties of two liquid mixtures with very different molecular scales proves the robustness of our study strategy, which could be used to study any liquid mixtures and understand their related physicochemical phenomena. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/25380 |
| Date | January 2020 |
| Creators | Li, Dongyang |
| Contributors | Xi, Li, Chemical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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